IPv6 Operations E. Davies
Internet-Draft Consultant
Expires: January 6, 2006 S. Krishnan
Ericsson
P. Savola
CSC/Funet
July 5, 2005
IPv6 Transition/Co-existence Security Considerationsdraft-ietf-v6ops-security-overview-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The transition from a pure IPv4 network to a network where IPv4 and
IPv6 co-exist brings a number of extra security considerations that
need to be taken into account when deploying IPv6 and operating the
dual-protocol network and the associated transition mechanisms. This
document attempts to give an overview of the various issues grouped
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Internet-Draft IPv6 Security Overview July 20051. Introduction
The transition from a pure IPv4 network to a network where IPv4 and
IPv6 co-exist brings a number of extra security considerations that
need to be taken into account when deploying IPv6 and operating the
dual-protocol network with its associated transition mechanisms.
This document attempts to give an overview of the various issues
grouped into three categories:
o issues due to the IPv6 protocol itself,
o issues due to transition mechanisms, and
o issues due to IPv6 deployment.
It is important to understand that we have to be concerned not about
replacing IPv4 with IPv6 (in the short term), but with adding IPv6 to
be operated in parallel with IPv4 [I-D.savola-v6ops-transarch].
This document also describes two matters which have been wrongly
identified as potential security concerns for IPv6 in the past and
explains why they are unlikely to cause problems: considerations
about probing/mapping IPv6 addresses (Appendix A), and considerations
with respect to privacy in IPv6 (Appendix B).
2. Issues Due to IPv6 Protocol2.1 IPv6 Protocol-specific Issues
There are significant differences between the features of IPv6 and
IPv4: some of these specification changes may result in potential
security issues. Several of these issues have been discussed in
separate drafts but are summarized here to avoid normative references
which may not become RFCs. The following specification-related
problems have been identified, but this is not necessarily a complete
list:
2.1.1 Routing Headers and Hosts
All IPv6 nodes must be able to process Routing Headers [RFC2460].
This RFC can be interpreted, although it is not clearly stated, to
mean that all nodes (including hosts) must have this processing
enabled. This can result in hosts forwarding received traffic if
there are segments left in the Routing Header when it arrives at the
host.
A number of potential security issues associated with this behavior
were documented in [I-D.savola-ipv6-rh-hosts]. Some of these issues
have been resolved (a separate routing header type is now used for
Mobile IPv6 [RFC3775] and ICMP Traceback has not been standardized),
but two issues remain:
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o Routing headers can be used to evade access controls based on
destination addresses. This could be achieved by sending a packet
ostensibly to a publicly accessible host address but with a
routing header containing a 'forbidden' address. If the publicly
accessible host is processing routing headers it will forward the
packet to the destination address in the routing header which
would have been forbidden by the packet filters if the address had
been in the destination field when the packet was checked.
o If the packet source address in the previous case can be spoofed,
any host could be used to mediate an anonymous reflection denial-
of-service attack by having any publicly accessible host redirect
the attack packets.
2.1.2 Routing Headers for Mobile IPv6 and Other Purposes
In addition to the basic Routing Header (Type 0), which is intended
to influence the trajectory of a packet through a network by
specifying a sequence of router 'waypoints', Routing Header (Type 2)
has been defined as part of the Mobile IPv6 specifications in
[RFC3775]. The Type 2 Routing Header is intended for use by hosts to
handle 'interface local' forwarding needed when packets are sent to
the care-of address of a mobile node which is away from its home
address.
It is important that nodes treat the different types of routing
header appropriately. It should be possible to apply separate
filtering rules to the different types of Routing Header. By design,
hosts must process Type 2 Routing Headers to support Mobile IPv6 but
routers should not: to avoid the issues in Section 2.1.1 it may be
desirable to forbid or limit the processing of Type 0 Routing Headers
in hosts and some routers.
Routing Headers are an extremely powerful and general capability.
Alternative future uses of Routing Headers need to be carefully
assessed to ensure that they do not open new avenues of attack that
can be exploited.
2.1.3 Site-scope Multicast Addresses
IPv6 supports multicast addresses with site scope which can
potentially allow an attacker to identify certain important resources
on the site if misused.
Particular examples are the 'all routers' (FF05::2) and 'all DHCP
servers' (FF05::1:3) addresses defined in [RFC2375]: an attacker that
is able to infiltrate a message destined for these addresses on to
the site will potentially receive in return information identifying
key resources on the site. This information can then be the target
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of directed attacks ranging from simple flooding to more specific
mechanisms designed to subvert the device.
Some of these addresses have current legitimate uses within a site.
The risk can be minimized by ensuring that all firewalls and site
boundary routers are configured to drop packets with site scope
destination addresses. Also nodes should not join multicast groups
for which there is no legitimate use on the site and site routers
should be configured to drop packets directed to these unused
addresses.
2.1.4 ICMPv6 and Multicast
It is possible to launch a denial-of-service (DoS) attack using IPv6
which could be amplified by the multicast infrastructure.
Unlike ICMP for IPv4, ICMPv6 [RFC2463] allows error notification
responses to be sent when certain unprocessable packets are sent to
multicast addresses.
The cases in which responses are sent are:
o The received packet is longer than the next link MTU: 'Packet Too
Big' responses are needed to support Path MTU Discovery for
multicast traffic.
o The received packet contains an unrecognized option in a hop-by-
hop or destination options extension header with the first two
bits of the option type set to binary '10': 'Parameter Problem'
responses are intended to inform the source that some or all of
the recipients cannot handle the option in question.
If an attacker can craft a suitable packet sent to a multicast
destination, it may be possible to elicit multiple responses directed
at the victim (the spoofed source of the multicast packet). On the
other hand, the use of 'reverse path forwarding' checks to eliminate
loops in multicast forwarding automatically limits the range of
addresses which can be spoofed.
In practice an attack using oversize packets is unlikely to cause
much amplification unless the attacker is able to carefully tune the
packet size to exploit a network with smaller MTU in the edge than
the core. Similarly a packet with an hop-by-hop option would be
dropped by the first router. However a packet with an destination
option could generate multiple responses.
In addition to amplification, this kind of attack would potentially
consume large amounts of forwarding state resources in routers on
multicast-enabled networks. These attacks are discussed in more
detail in [I-D.savola-v6ops-firewalling].
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Internet-Draft IPv6 Security Overview July 20052.1.5 Anycast Traffic Identification and Security
IPv6 introduces the notion of anycast addresses and services.
Originally the IPv6 standards disallowed using an anycast address as
the source address of a packet. Responses from an anycast server
would therefore supply a unicast address for the responding server.
To avoid exposing knowledge about the internal structure of the
network, it is recommended that anycast servers now take advantage of
the ability to return responses with the anycast address as the
source address if possible.
If the server needs to use a unicast address for any reason, it may
be desirable to consider using specialized addresses for anycast
servers which are not used for any other part of the network to
restrict the information exposed. Alternatively operators may wish
to restrict the use of anycast services from outside the domain, thus
requiring firewalls to filter anycast requests. For this purpose,
firewalls need to know which addresses are being used for anycast
services: these addresses are arbitrary and not distinguishable from
any other IPv6 unicast address by structure or pattern.
One particular class of anycast addresses that should be given
special attention is the set of Subnet-Router anycast addresses
defined in The IPv6 Addressing Architecture [RFC3513]. All routers
are required to support these addresses for all subnets for which
they have interfaces. For most subnets using global unicast
addresses, filtering anycast requests to these addresses can be
achieved by dropping packets with the lower 64 bits (the Interface
Identifier) set to all zeroes.
2.1.6 Address Privacy Extensions Interact with DDoS Defenses
The purpose of the privacy extensions for stateless address auto-
configuration [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] is to change
the interface identifier (and hence the global scope addresses
generated from it) from time to time. By varying the addresses used,
eavesdroppers and other information collectors find it more difficult
to identify which transactions actually relate to a specific node.
A security issue may result from this if the frequency of node
address change is sufficiently great to achieve the intended aim of
the privacy extensions: with a relatively high rate of change, the
observed behavior of the node could look very like that of a
compromised node which was the source of a distributed denial of
service (DDoS). It would thus be difficult to for any future
defenses against DDoS attacks to distinguish between a high rate of
change of addresses resulting from genuine use of the privacy
extensions and a compromised node being used as the source of a DDoS
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with 'in-prefix' spoofed source addresses as described in
[I-D.dupont-ipv6-rfc3041harmful].
Even if a node is well behaved, the change in the address could make
it harder for a security administrator to define a policy rule (e.g.
access control list) that takes into account a specific node.
2.1.7 Dynamic DNS: Stateless Address Auto-Configuration, Privacy Extensions and SEND
The introduction of Stateless Address Auto-Configuration (SLAAC)
[RFC2462] with IPv6 provides an additional challenge to the security
of Dynamic DNS (DDNS). With manual addressing or the use of DHCP,
the number of security associations that need to be maintained to
secure access to the DNS server is limited, assuming any necessary
updates are carried out by the DHCP server. This is true equally for
IPv4 and IPv6.
Since SLAAC does not make use of a single and potentially trusted
DHCP server, but depends on the node obtaining the address, securing
the insertion of updates into DDNS may need a security association
between each node and the DDNS server. This is discussed further in
[I-D.ietf-dnsop-ipv6-dns-issues].
Using the Privacy Extensions to SLAAC [RFC3041][I-D.ietf-ipv6-
privacy-addrs-v2] may significantly increase the rate of updates of
DDNS. Even if a node using the Privacy Extensions does not publish
its address for 'forward' lookup (as that would effectively
compromise the privacy which it is seeking), it may still need to
update the reverse DNS records so that reverse routability checks can
be carried out. If the rate of change needed to achieve real privacy
has to be increased as is mentioned in Section 2.1.6 the update rate
for DDNS may be excessive.
Similarly, the cryptographically generated addresses used by SEND
[RFC3971] are expected to be periodically regenerated in line with
recommendations for maximum key lifetimes. This regeneration could
also impose a significant extra load on DDNS.
2.1.8 Extension Headers
A number of issues relating to the specification of IPv6 Extension
headers have been identified. Several of these are discussed in
[I-D.savola-v6ops-firewalling].
2.1.8.1 Processing Extension Headers in Middleboxes
In IPv4 deep packet inspection techniques are used to implement
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policing and filtering both as part of routers and in middleboxes
such as firewalls. Fully extending these techniques to IPv6 would
require inspection of all the extension headers in a packet. This is
essential to ensure that policy constraints on the use of certain
headers and options are enforced and to remove, at the earliest
opportunity, packets containing potentially damaging unknown options.
This requirement appears to conflict with Section 4 of the IPv6
specification in [RFC2460] which requires that destination options
are not processed at all until the packet reaches the appropriate
destination (either the final destination or a routing header
waypoint).
Also [RFC2460] forbids processing the headers other than in the order
in which they appear in the packet.
A further ambiguity relates to whether an intermediate node should
discard a packet which contains a header or destination option which
it does not recognize. If the rules above are followed slavishly, it
is not (or may not be) legitimate for the intermediate node to
discard the packet because it should not be processing those headers
or options.
[RFC2460] therefore does not appear to take account of the behavior
of middleboxes and other non-final destinations which may be
inspecting the packet, and thereby potentially limits the security
protection of these boxes.
2.1.8.2 Processing Extension Header Chains
There is a further problem for middleboxes that want to examine the
transport headers which are located at the end of the IPv6 header
chain. In order to locate the transport header or other protocol
data unit, the node has to parse the header chain.
The IPv6 specification [RFC2460] does not mandate the use of the
Type-Length-Value format with a fixed layout for the start of each
header although it is used for the majority of headers currently
defined. (Only the Type field is guaranteed in size and offset).
A middlebox cannot therefore guarantee to be able to process header
chains which may contain headers defined after the box was
manufactured. As noted in Section 2.1.8.1, middleboxes ought not to
have to know about all header types in use but still need to be able
to skip over such headers to find the transport PDU start. This
either limits the security which can be applied in firewalls or makes
it difficult to deploy new extension header types.
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At the time of writing, only the Fragment Header does not fully
conform to the TLV format used for other extension headers. In
practice, many firewalls reconstruct fragmented packets before
performing deep packet inspection, so this divergence is less
problematic than it might have been, and is at least partially
justified because the full header chain is not present in all
fragments.
Destination Options may also contain unknown options. However, the
options are encoded in TLV format so that intermediate nodes can skip
over them during processing, unlike the enclosing extension headers.
2.1.8.3 Unknown Headers/Destination Options and Security Policy
A strict security policy might dictate that packets containing either
unknown headers or destination options are discarded by firewalls or
other filters. This requires the firewall to process the whole
extension header chain which may be currently in conflict with the
IPv6 specification as discussed in Section 2.1.8.1.
Even if the firewall does inspect the whole header chain, it may not
be sensible to discard packets with items by the firewall: the
intermediate node has no knowledge of which options and headers are
implemented in the destination node. Hence it is highly desirable to
make the discard policy configurable. This will avoid firewalls
dropping packets with legitimate items that they do not recognize
because their hardware or software is not aware of a new definition.
2.1.8.4 Excessive Hop-by-Hop Options
IPv6 does not limit the number of hop by hop options which can be
present in a hop-by-hop option header. The lack of a limit can be
used to mount denial of service attacks affecting all nodes on a path
as described in [I-D.krishnan-ipv6-hopbyhop].
2.1.8.5 Overuse of Router Alert Option
The IPv6 router alert option specifies a hop-by-hop option that, if
present, signals the router to take a closer look at the packet.
This can be used for denial of service attacks. By sending a large
number of packets containing a router alert option an attacker can
deplete the processor cycles on the routers available to legitimate
traffic.
2.1.9 Fragmentation: Reassembly and Deep Packet Inspection
The current specifications of IPv6 in [RFC2460] do not mandate any
minimum packet size for the fragments of a packet before the last
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one, except for the need to carry the unfragmentable part in all
fragments.
The unfragmentable part does not include the transport port numbers
so that it is possible that the first fragment does not contain
sufficient information to carry out deep packet inspection involving
the port numbers.
Also the reassembly rules for fragmented packets in [RFC2460] do not
mandate behavior which would minimize the effects of overlapping
fragments.
Depending on the implementation of packet reassembly and the
treatment of packet fragments in firewalls and other nodes which use
deep packet inspection for traffic filtering, this potentially leaves
IPv6 open to the sort of attacks described in [RFC1858] and [RFC3128]
for IPv4.
There is no reason to allow overlapping packet fragments and overlaps
could be prohibited in a future revision of the protocol
specification. Some implementations already drop all packets with
overlapped fragments.
Specifying a minimum size for packet fragments does not help in the
same way as it does for IPv4 because IPv6 extension headers can be
made to appear very long: an attacker could insert one or more
undefined destination options with long lengths and the 'ignore if
unknown' bit set. Given the guaranteed minimum MTU of IPv6 it seems
reasonable that hosts should be able to ensure that the transport
port numbers are in the first fragment in almost all cases and that
deep packet inspection should be very suspicious of first fragments
that do not contain them.
2.1.10 Fragmentation Related DoS Attacks
Packet reassembly in IPv6 hosts also opens up the possibility of
various fragment-related security attacks. Some of these are
analogous to attacks identified for IPv4. Of particular concern is a
DoS attack based on sending large numbers of small fragments without
a terminating last fragment which would potentially overload the
reconstruction buffers and consume large amounts of CPU resources.
Mandating the size of packet fragments could reduce the impact of
this kind of attack by limiting the rate at which fragments could
arrive and limiting the number of fragments which need to be
processed.
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Internet-Draft IPv6 Security Overview July 20052.1.11 Link-Local Addresses and Securing Neighbor Discovery
All IPv6 nodes are required to configure a link-local address on each
interface. This address is used to communicate with other nodes
directly connected to the link accessed via the interface, especially
during the neighbor discovery and auto-configuration processes.
Link-local addresses are fundamental to the operation of the Neighbor
Discovery Protocol (NDP) [RFC2461] and SLAAC [RFC2462]. NDP also
provides the functionality of associating link layer and IP addresses
provided by the Address Resolution Protocol (ARP) in IPv4 networks.
The standard version of NDP is subject to a number of security
threats related to ARP spoofing attacks on IPv4. These threats have
been documented in [RFC3756] and mechanisms to combat them specified
in SEcure Neighbor Discovery (SEND) [RFC3971]. SEND is an optional
mechanism which is particularly applicable to wireless and other
environments where it is difficult to physically secure the link.
Because the link-local address can, by default, be acquired without
external intervention or control, it allows an attacker to commence
communication on the link without needing to acquire information
about the address prefixes in use or communicate with any authorities
on the link. This feature gives a malicious node the opportunity to
mount an attack on any other node which is attached to this link;
this vulnerability exists in addition to possible direct attacks on
NDP. Link-local addresses may also facilitate the unauthorized use
of the link bandwidth ('bandwidth theft') to communicate with another
unauthorized node on the same link.
Link-local addresses allocated from the prefix 169.254.0.0/16 are
available in IPv4 as well and procedures for using them are described
in [I-D.ietf-zeroconf-ipv4-linklocal] but the security issues were
not as pronounced as for IPv6 for the following reasons:
o link-local addresses are not mandatory in IPv4 and are primarily
intended for isolated or ad hoc networks that cannot acquire a
routable IPv4 address by other means,
o IPv4 addresses are not universally supported across operating
systems, and
o the IPv4 link-local address should be removed when a non-link-
local address is configured on the interface and will generally
not be allocated unless other means of acquiring an address are
not available.
These vulnerabilities can be mitigated in several ways. A general
solution will require
o authenticating the link layer connectivity, for example by using
IEEE 802.1x functionality, port-based MAC address security
(locking), or physical security, and
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o using SEcure Neighbor Discovery (SEND) to create a
cryptographically generated link-local address as described in
[RFC3971] which is tied to the authenticated link layer address.
This solution would be particularly appropriate in wireless LAN
deployments where it is difficult to physically secure the
infrastructure
In wired environments, where the physical infrastructure is
reasonably secure, it may be sufficient to ignore communication
requests originating from a link-local address for other than local
network management purposes. This requires that nodes should only
accept packets with link-local addresses for a limited set of
protocols including NDP, MLD and other functions of ICMPv6.
2.1.11.1 Securing Router Advertisements
As part of the Neighbor Discovery process, routers on a link
advertise their capabilities in Router Advertisement messages. The
version of NDP defined in [RFC2461] does not protect the integrity of
these messages or validate the assertions made in the messages with
the result that any node which connects to the link can maliciously
claim to offer routing services which it will not fulfill, and
advertise inappropriate prefixes and parameters. These threats have
been documented in [RFC3756].
SEND [RFC3971] can be used to provide verification that routers are
authorized to provide the services they advertise through a
certificate-based mechanism. This capability of SEND is also
particularly appropriate for wireless environments where clients are
reliant on the assertions of the routers rather than a physically
secured connection.
2.1.12 Mobile IPv6
Mobile IPv6 offers significantly enhanced security compared with
Mobile IPv4 especially when using optimized routing and care-of
addresses. Return routability checks are used to provide relatively
robust assurance that the different addresses which a mobile node
uses as it moves through the network do indeed all refer to the same
node. The threats and solutions are described in [RFC3775] and a
more extensive discussion of the security aspects of the design can
be found in [I-D.ietf-mip6-ro-sec].
2.1.12.1 Obsolete Home Address Option in Mobile IPv6
The Home Address option specified in early drafts of Mobile IPv6
would have allowed a trivial source spoofing attack: hosts were
required to substitute the source address of incoming packets with
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the address in the option, thereby potentially evading checks on the
packet source address. This is discussed at greater length in
[I-D.savola-ipv6-rh-ha-security]. The version of Mobile IPv6 as
standardized in [RFC3775] has removed this issue by ensuring that the
Home Address destination option is only processed if there is a
corresponding binding cache entry and securing Binding Update
messages.
A number of pre-standard implementations of Mobile IPv6 were
available which implemented this obsolete and insecure option: care
should be taken to avoid running such obsolete systems.
2.2 IPv4-mapped IPv6 Addresses
Overloaded functionality is always a double-edged sword: it may yield
some deployment benefits, but often also incurs the price which comes
with ambiguity.
One example of such is IPv4-mapped IPv6 addresses: a representation
of an IPv4 address as an IPv6 address inside an operating system.
Since the original specification, the use of IPv4-mapped addresses
has been extended to a transition mechanism, Stateless IP/ICMP
Translation algorithm (SIIT) [RFC2765], where they are potentially
used in the addresses of packets on the wire.
Therefore, it becomes difficult to unambiguously discern whether an
IPv4 mapped address is really an IPv4 address represented in the IPv6
address format *or* an IPv6 address received from the wire (which may
be subject to address forgery, etc.).
In addition, special cases like these, while giving deployment
benefits in some areas, require a considerable amount of code
complexity (e.g. in the implementations of bind() system calls and
reverse DNS lookups) which is probably undesirable. Some of these
issues are discussed in [I-D.cmetz-v6ops-v4mapped-api-harmful] and
[I-D.itojun-v6ops-v4mapped-harmful].
In practice, although the packet translation mechanisms of SIIT are
specified for use in the Network Address Translator - Protocol
Translator (NAT-PT) [RFC2765], NAT-PT uses a mechanism different from
IPv4-mapped IPv6 addresses for communicating embedded IPv4 addresses
in IPv6 addresses. Also SIIT is not recommended for use as a
standalone transition mechanism. Given the issues that have been
identified, it seems appropriate that mapped addresses should not be
used on the wire. However, changing application behavior by
deprecating the use of mapped addresses in the operating system
interface would have significant impact on application porting
methods [RFC4038]and needs further study.
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Internet-Draft IPv6 Security Overview July 20052.3 Increased End-to-End Transparency
One of the major design aims of IPv6 has been to maintain the
original IP architectural concept of end-to-end transparency.
Transparency can help foster technological innovation in areas such
as peer-to-peer communication but maintaining the security of the
network at the same time requires some modifications in the network
architecture. Ultimately, it is also likely to need changes in the
security model as compared with the norms for IPv4 networks.
2.3.1 IPv6 Networks without NATs
The necessity of introducing Network Address Translators (NATs) into
IPv4 networks, resulting from a shortage of IPv4 addresses, has
removed the end-to-end transparency of most IPv4 connections: the use
of IPv6 would restore this transparency. However, the use of NATs,
and the associated private addressing schemes, has become
inappropriately linked to the provision of security in enterprise
networks. The restored end-to-end transparency of IPv6 networks can
therefore be seen as a threat by poorly informed enterprise network
managers. Some seem to want to limit the end-to-end capabilities of
IPv6, for example by deploying private, local addressing and
translators, even when it is not necessary because of the abundance
of IPv6 addresses.
Recommendations for designing an IPv6 network to meet the perceived
security and connectivity requirements implicit in the current usage
of IPv4 NATs whilst maintaining the advantages of IPv6 end-to-end
transparency are described in IPv6 Network Architecture Protection
[I-D.ietf-v6ops-nap].
2.3.2 Enterprise Network Security Model for IPv6
The favored model for enterprise network security in IPv4 stresses
the use of a security perimeter policed by autonomous firewalls and
incorporating the NATs. Both perimeter firewalls and NATs introduce
asymmetry and reduce the transparency of communications through these
perimeters. The symmetric bidirectionality and transparency which
are extolled as virtues of IPv6 may seem to be at odds with this
model. Consequently network managers may even see them as
undesirable attributes, in conflict with their need to control
threats to and attacks on the networks they administer.
It is worth noting that IPv6 does not *require* end-to-end
connectivity. It merely provides end-to-end addressability; the
connectivity can still be controlled using firewalls (or other
mechanisms), and it is indeed wise to do so.
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A number of matters indicate that IPv6 networks should migrate
towards an improved security model, which will increase the overall
security of the network but facilitate end-to-end communication:
o Increased usage of end-to-end security especially at the network
layer. IPv6 mandates the provision of IPsec capability in all
nodes and increasing usage of end-to-end security is a challenge
to current autonomous firewalls that are unable to perform deep
packet inspection on encrypted packets. It is also incompatible
with NATs because they modify the packets, even when packets are
only authenticated rather than encrypted.
o Acknowledgement that over-reliance on the perimeter model is
potentially dangerous. An attacker who can penetrate today's
perimeters will have free rein within the perimeter, in many
cases. Also a successful attack will generally allow the attacker
to capture information or resources and make use of them.
o Development of mechanisms such as 'Trusted Computing' which will
increase the level of trust which network managers are able to
place on hosts.
o Development of centralized security policy repositories and secure
distribution mechanisms which, in conjunction with trusted hosts,
will allow network managers to place more reliance on security
mechanisms at the end points. The mechanisms are likely to
include end-node firewalling and intrusion detection systems as
well as secure protocols that allow end points to influence the
behavior of perimeter security devices.
o Review of the role of perimeter devices with increased emphasis on
intrusion detection, network resource protection and coordination
to thwart distributed denial of service attacks.
Several of the technologies required to support an enhanced security
model are still under development, including secure protocols to
allow end points to control firewalls: the complete security model
utilizing these technologies is now emerging but still requires some
development.
In the meantime, initial deployments will need to make use of similar
firewalling and intrusion detection techniques to IPv4 which may
limit end-to-end transparency temporarily, but should be prepared to
use the new security model as it develops and avoid the use of NATs
by the use of the architectural techniques described in [I-D.ietf-
v6ops-nap]. In particular, using NAT-PT [RFC2766] as a general
purpose transition mechanism should be avoided as it is likely to
limit the exploitation of end-to-end security and other IPv6
capabilities in future as explained in [I-D.ietf-v6ops-natpt-to-
exprmntl].
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Internet-Draft IPv6 Security Overview July 20053. Issues Due to Transition Mechanisms3.1 IPv6 Transition/Co-existence Mechanism-specific Issues
The more complicated the IPv6 transition/co-existence becomes, the
greater the danger that security issues will be introduced either
o in the mechanisms themselves,
o in the interaction between mechanisms, or
o by introducing unsecured paths through multiple mechanisms.
These issues may or may not be readily apparent. Hence it would be
desirable to keep the mechanisms simple, as few in number as possible
and built from as small pieces as possible to simplify analysis.
One case where such security issues have been analyzed in detail is
the 6to4 tunneling mechanism [RFC3964].
As tunneling has been proposed as a model for several more cases than
are currently being used, its security properties should be analyzed
in more detail. There are some generic dangers to tunneling:
o it may be easier to avoid ingress filtering checks
o it is possible to attack the tunnel interface: several IPv6
security mechanisms depend on checking that Hop Limit equals 255
on receipt and that link-local addresses are used. Sending such
packets to the tunnel interface is much easier than gaining access
to a physical segment and sending them there.
o automatic tunneling mechanisms are typically particularly
dangerous as there is no pre-configured association between end
points. Accordingly, at the receiving end of the tunnel packets
have to be accepted and decapsulated from any source.
Consequently, special care should be taken when specifying
automatic tunneling techniques.
3.2 Automatic Tunneling and Relays
Two mechanisms have been (or are being) specified which use automatic
tunneling and are intended for use outside a single domain. These
mechanisms encapsulate the IPv6 packet directly in an IPv4 packet in
the case of 6to4 [RFC3056] or in an IPv4 UDP packet in the case of
Teredo [I-D.huitema-v6ops-teredo]. In each case packets can be sent
and received by any similarly equipped nodes in the IPv4 Internet.
As mentioned in Section 3.1, a major vulnerability in such approaches
is that receiving nodes must allow decapsulation of traffic sourced
from anywhere in the Internet. This kind of decapsulation function
must be extremely well secured because of the wide range of potential
sources.
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An even more difficult problem is how these mechanisms are able to
establish communication with native IPv6 nodes or between the
automatic tunneling mechanisms: such connectivity requires the use of
some kind of "relay". These relays could be deployed in various
locations such as:
o all native IPv6 nodes,
o native IPv6 sites,
o in IPv6-enabled ISPs, or
o just somewhere in the Internet.
Given that a relay needs to trust all the sources (e.g., in the 6to4
case, all 6to4 routers) which are sending it traffic, there are
issues in achieving this trust and at the same time scaling the relay
system to avoid overloading a small number of relays.
As authentication of such a relay service is very difficult to
achieve, and particularly so in some of the possible deployment
models, relays provide a potential vehicle for address spoofing,
(reflected) Denial-of-Service attacks, and other threats.
Threats related to 6to4 and measures to combat them are discussed in
[RFC3964]. [I-D.huitema-v6ops-teredo] incorporates extensive
discussion of the threats to Teredo and measures to combat them.
3.3 Tunneling May Break Security Assumptions
NATs and firewalls have been deployed extensively in the IPv4
Internet, as discussed in Section 2.3. Operators who deploy them
typically have some security/operational requirements in mind (e.g. a
desire to block inbound connection attempts), which may or may not be
misguided.
The addition of tunneling can change the security model which such
deployments are seeking to enforce. IPv6-over-IPv4 tunneling using
protocol 41 is typically either explicitly allowed, or disallowed
implicitly. Tunneling IPv6 over IPv4 encapsulated in UDP constitutes
a more difficult problem as UDP must usually be allowed to pass
through NATs and firewalls. Consequently, using UDP implies the
ability to punch holes in NAT's and firewalls although, depending on
the implementation, this ability may be limited or only achieved in a
stateful manner. In practice, the mechanisms have been explicitly
designed to traverse both NATs and firewalls in a similar fashion.
One possible view is that use of tunneling is especially questionable
in home/SOHO environments where the level of expertise in network
administration is typically not very high; in these environments the
hosts may not be as tightly managed as in others (e.g., network
services might be enabled unnecessarily), leading to possible
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security break-ins or other vulnerabilities.
Holes can be punched both intentionally and unintentionally. In
cases where the administrator or user makes an explicit decision to
create the hole, this is less of a problem, although (for example)
some enterprises might want to block IPv6 tunneling explicitly if
employees were able to create such holes without reference to
administrators. On the other hand, if a hole is punched
transparently, it is likely that a proportion of users will not
understand the consequences: this will very probably result in a
serious threat sooner or later.
When deploying tunneling solutions, especially tunneling solutions
which are automatic and/or can be enabled easily by users who do not
understand the consequences, care should be taken not to compromise
the security assumptions held by the users.
For example, NAT traversal should not be performed by default unless
there is a firewall producing a similar by-default security policy to
that provided by IPv4 NAT. IPv6-in-IPv4 (protocol 41) tunneling is
less of a problem, as it is easier to block if necessary; however, if
the host is protected in IPv4, the IPv6 side should be protected as
well.
As has been shown in Appendix A, it is relatively easy to determine
the IPv6 address corresponding to an IPv4 address in tunneling
deployments. It is therefore vital NOT to rely on "security by
obscurity" i.e., assuming that nobody is able to guess or determine
the IPv6 address of the host especially when using automatic
tunneling transition mechanisms.
4. Issues Due to IPv6 Deployment4.1 IPv6 Service Piloting Done Insecurely
In many cases, IPv6 service piloting is done in a manner which is
less secure than can be achieved for an IPv4 production service. For
example, hosts and routers might not be protected by IPv6 firewalls,
even if the corresponding IPv4 service is fully protected by
firewalls.
The other possible alternative, in some instances, is that no service
piloting is permitted because IPv6 firewalls and other security
capabilities, such as intrusion detection systems may not be widely
available. Consequently, IPv6 deployment suffers and expertise
accumulates less rapidly.
These problems may be partly due to the relatively slow development
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and deployment of IPv6-capable firewall equipment, but there is also
a lack of information: actually, there are quite a few IPv6 packet
filters and firewalls already in existence, which could be used for
provide sufficient access controls, but network administrators may
not be aware of them yet and there is a lack of documented
operational practice.
However, there appears to be a real lack in the area of 'personal
firewalls'. Also enterprise firewalls are at an early stage of
development and may not provide all the capabilities needed to
implement the necessary IPv6 filtering rules. The same devices that
support and are used for IPv4 today are often expected to also become
IPv6-capable -- even though this is not really required and the
equipment may not have the requisite hardware capabilities to support
fast packet filtering for IPv6. That is, IPv4 access could be
filtered by one firewall, and when IPv6 access is added, it could be
protected by another firewall; they don't have to be the same box,
and even their models don't have to be the same.
A lesser factor may be that some design decisions in the IPv6
protocol make it more difficult for firewalls to be implemented and
work in all cases and to be fully future proof (e.g. when new
extension headers are used) as discussed in Section 2.1.8: it is
significantly more difficult for intermediate nodes to process the
IPv6 header chains than IPv4 packets.
A similar argument, which is often quoted as hindering IPv6
deployment, has been the lack of Intrusion Detection Systems (IDS).
It is not clear whether this is more of an excuse than a real reason.
4.2 Enabling IPv6 by Default Reduces Usability
A practical disadvantage of enabling IPv6 at the time of writing is
that it typically reduces the observed service level by a small
fraction; that is, the usability suffers.
There are at least three factors contributing to this effect:
o global IPv6 routing is still rather unstable, leading to packet
loss, lower throughput, and higher delay (this was discussed with
respect to the experimental 6Bone network in [I-D.savola-v6ops-
6bone-mess])
o some applications cannot properly handle both IPv4 and IPv6 or may
have problems handling all the fallbacks and failure modes (and in
some cases, e.g. if the TCP timeout kicks in, this may result in
very poor performance)
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o some DNS server implementations have flaws that severely affect
DNS queries for IPv6 addresses as discussed in [I-D.ietf-dnsop-
misbehavior-against-aaaa]
Actually, some providers are fully ready to offer IPv6 services (e.g.
web) today, but because that would (or, at least, might) result in
problems for many of their customers or users who are, by default,
using active dual-stack systems the services are not turned on: as a
compromise, the services are often published under a separate domain
or subdomain, and are, in practice, not much used as a consequence.
These issues are also described at some length in [I-D.ietf-v6ops-
v6onbydefault].
An additional problem is the limited implementation of high
availability capabilities supporting IPv6. In particular,
development of the IPv6 version of the Virtual Router Redundancy
Protocol (VRRP) [I-D.ietf-vrrp-ipv6-spec] has lagged the development
of the main IPv6 protocol although alternatives may be available for
some environments.
4.3 Addressing Schemes and Securing Routers
Whilst in general terms brute force scanning of IPv6 subnets is
essentially impossible due to the enormously larger address space of
IPv6 and the 64 bit interface identifiers (see Appendix A), this will
be obviated if administrators do not take advantage of the large
space to use unguessable interface identifiers.
Because the unmemorability of complete IPv6 addresses there is a
temptation for administrators to use small integers as interface
identifiers when manually configuring them, as might happen on point-
to-point links. Such allocations make it easy for an attacker to
find active nodes that they can then port scan.
To make use of the larger address space properly, administrators
should be very careful when entering IPv6 addresses in their
configurations (e.g. Access Control List), since numerical IPv6
addresses are more prone to human error than IPv4 due to their length
and unmemorability.
It is also essential to ensure that the management interfaces of
routers are well secured as the router will usually contain a
significant cache of neighbor addresses in its neighbor cache.
4.4 Consequences of Multiple Addresses in IPv6
One positive consequence of IPv6 is that nodes which do not require
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global access can communicate locally just by the use of a link-local
address (if very local access is sufficient) or across the site by
using a Unique Local Address (ULA). In either case it is easy to
ensure that access outside the assigned domain of activity can be
controlled by simple filters (which may be the default for link-
locals). However, the security hazards of using link-local addresses
for non-management purposes as documented in Section 2.1.11 should be
borne in mind.
On the other hand, the possibility that a node or interface can have
multiple global scope addresses makes access control filtering both
on ingress and egress more complex and requires higher maintenance
levels.
The addresses could be from the same network prefix (for example,
privacy mechanisms [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] will
periodically create new addresses taken from the same prefix and two
or more of these may be active at the same time), or from different
prefixes (for example, when a network is multihomed or is
implementing anycast services). In either case, it is possible that
a single host could be using several different addresses with
different prefixes. It would be desirable that the Security
Administrator should be able to identify that the same host is behind
all these addresses.
4.5 Deploying ICMPv6
In IPv4 it is commonly accepted that some filtering of ICMP packets
by firewalls is essential to maintain security. Because of the
extended use that is made of ICMPv6 [RFC2461] with a multitude of
functions, the simple set of dropping rules that are usually applied
in IPv4 need to be significantly developed for IPv6. The blanket
dropping of all ICMP messages that is used in some very strict
environments is simply not possible for IPv6.
In an IPv6 firewall, policy needs to allow some messages through the
firewall but also has to permit certain messages to and from the
firewall, especially those with link-local sources on links to which
the firewall is attached. These messages must be permitted to ensure
that Neighbor Discovery [RFC2462], Multicast Listener Discovery
[RFC2710], [RFC3810] and Stateless Address Configuration [RFC2463]
work as expected.
Recommendations for filtering ICMPv6 messages can be found in
[I-D.davies-v6ops-icmpv6-filtering-bcp].
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Internet-Draft IPv6 Security Overview July 20054.5.1 Problems Resulting from ICMPv6 Transparency
As described in Section 4.5, certain ICMPv6 error packets need to be
passed through a firewall in both directions. This means that some
ICMPv6 error packets can be exchanged between inside and outside
without any filtering.
Using this feature, malicious users can communicate between the
inside and outside of a firewall bypassing the administrator's
inspection (proxy, firewall etc.). For example in might be possible
to carry out a covert conversation through the payload of ICMPv6
error messages or tunnel inappropriate encapsulated IP packets in
ICMPv6 error messages. This problem can be alleviated by filtering
ICMPv6 errors using a stateful packet inspection mechanism to ensure
that the packet carried as a payload is associated with legitimate
traffic to or from the protected network.
4.6 IPsec Transport Mode
IPsec provides security to end-to-end communications at the network
layer (layer 3). The security features available include access
control, connectionless integrity, data origin authentication,
protection against replay attacks, confidentiality, and limited
traffic flow confidentiality (see [RFC2401] section 2.1). IPv6
mandates the implementation of IPsec in all conforming nodes, making
the usage of IPsec to secure end-to-end communication possible in a
way which is generally not available to IPv4.
To secure IPv6 end-to-end communications, IPsec transport mode would
generally be the solution of choice. However, use of these IPsec
security features can result in novel problems for network
administrators and decrease the effectiveness of perimeter firewalls
because of the increased prevalence of encrypted packets on which the
firewalls cannot perform deep packet inspection and filtering.
One example of such problems is the lack of security solutions in the
middlebox, including effective content-filtering, ability to provide
DoS prevention based on the expected TCP protocol behavior, and
intrusion detection. Future solutions to this problem are discussed
in Section 2.3.2. Another example is an IPsec-based DoS (e.g.,
sending malformed ESP/AH packets) which can be especially detrimental
to software-based IPsec implementations.
4.7 Reduced Functionality Devices
With the deployment of IPv6 we can expect the attachment of a very
large number of new IPv6-enabled devices with scarce resources and
low computing capacity. The resource limitations are generally
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because of a market requirement for cost reduction. Some such
devices may not be able even to perform the minimum set of functions
required to protect themselves (e.g. 'personal' firewall, automatic
firmware update, enough CPU power to endure DoS attacks). This means
a different security scheme may be necessary for such embedded
devices.
4.8 Operational Factors when Enabling IPv6 in the Network
There are a number of reasons which make it essential to take
particular care when enabling IPv6 in the network equipment:
Initially, IPv6-enabled router software may be less stable than
current IPv4-only implementations and there is less experience with
configuring IPv6 routing, which can result in disruptions to the IPv6
routing environment and (IPv6) network outages.
IPv6 processing may not happen at (near) line speed (or at a
comparable performance level to IPv4 in the same equipment). A high
level of IPv6 traffic (even legitimate, e.g. Network News Transport
Protocol, NNTP) could easily overload IPv6 processing especially when
it is software-based without the hardware support typical in high-end
routers. This may potentially have deleterious knock-on effects on
IPv4 processing, affecting availability of both services.
Accordingly, if people don't feel confident enough in the IPv6
capabilities of their equipment, they will be reluctant to enable it
in their "production" networks.
Sometimes essential features may be missing from early releases of
vendors' software; an example is provision of software enabling IPv6
telnet/SSH access (e.g., to the configuration application of a
router), but without the ability to turn it off or limit access to
it!
Sometimes the default IPv6 configuration is insecure. For example,
in one vendor's implementation, if you have restricted IPv4 telnet to
only a few hosts in the configuration, you need to be aware that IPv6
telnet will be automatically enabled, that the configuration commands
used previously do not block IPv6 telnet, IPv6 telnet is open to the
world by default, and that you have to use a separate command to also
lock down the IPv6 telnet access.
Many operator networks have to run interior routing protocols for
both IPv4 and IPv6. It is possible to run the both in one routing
protocol, or have two separate routing protocols; either approach has
its tradeoffs [RFC4029]. If multiple routing protocols are used, one
should note that this causes double the amount of processing when
links flap or recalculation is otherwise needed -- which might more
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easily overload the router's CPU, causing slightly slower convergence
time.
4.9 Ingress Filtering Issues Due to Privacy Addresses
[RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] describes a method for
creating temporary addresses on IPv6 nodes to address privacy issues
created by the use of a constant identifier. In a network, which
implements such a mechanism, with a large number of nodes, new
temporary addresses may be created at a fairly high rate. This might
make it hard for ingress filtering mechanisms to distinguish between
legitimately changing temporary addresses and spoofed source
addresses, which are "in-prefix" (They use a topologically correct
prefix and non-existent interface ID). This can be addressed by
using finer grained access control mechanisms on the network egress
point.
4.10 Security Issues Due to ND Proxies
In order to span a single subnet over multiple physical links, a new
capability is being introduced in IPv6 to proxy Neighbor Discovery
messages. This node will be called an NDProxy (see [I-D.ietf-ipv6-
ndproxy]. NDProxies are susceptible to the same security issues as
the ones faced by hosts using unsecured Neighbor Discovery or ARP.
These proxies may process unsecured messages, and update the neighbor
cache as a result of such processing, thus allowing a malicious node
to divert or hijack traffic. This may undermine the advantages of
using SEND [RFC3971].
To resolve the security issues introduced by NDProxies, SEND needs to
be extended to be NDProxy aware.
5. IANA Considerations
This memo does not contain any actions for IANA.
6. Security Considerations
This memo attempts to give an overview of security considerations of
the different aspects of IPv6.
7. Acknowledgements
Alain Durand, Alain Baudot, Luc Beloeil, Andras Kis-Szabo, Alvaro
Vives, Janos Mohacsi and Mark Smith provided feedback to improve this
memo. SUSZUKI Shinsuke provided a number of additional issues in
cooperation with the Deployment Working Group of the Japanese IPv6
Promotion Council. Michael Wittsend and Michael Cole discussed
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Given the relatively immature state of IPv6 network security, if an
attacker knows the IPv4 address of the node and believes it to be
dual-stacked with IPv4 and IPv6, he might want to try to probe the
corresponding IPv6 address, based on the assumption that the security
defenses might be lower. This might be the case particularly for
nodes which are behind a NAT in IPv4, but globally addressable in
IPv6. Naturally, this is not a concern if similar and adequate
security policies are in place.
On the other hand, brute-force scanning or probing of addresses is
computationally infeasible due to the large search space of interface
identifiers on most IPv6 subnets (somewhat less than 64 bits wide,
depending on how identifiers are chosen), always provided that
identifiers are chosen at random out of the available space, as
discussed in [I-D.chown-v6ops-port-scanning-implications].
For example, automatic tunneling mechanisms typically use
deterministic methods for generating IPv6 addresses, so probing/
port-scanning an IPv6 node is simplified. The IPv4 address is
embedded at least in 6to4, Teredo and ISATAP addresses.
Additionally, it is possible (in the case of 6to4 in particular) to
learn the address behind the prefix; for example, Microsoft 6to4
implementation uses the address 2002:V4ADDR::V4ADDR while older Linux
and FreeBSD implementations default to 2002:V4ADDR::1. This could
also be used as one way to identify an implementation and hence
target any specific weaknesses.
One proposal has been to randomize the addresses or subnet identifier
in the address of the 6to4 router. This does not really help, as the
6to4 router (whether a host or a router) will return an ICMPv6 Hop
Limit Exceeded message, revealing the IP address. Hosts behind the
6to4 router can use methods such as RFC 3041 addresses to conceal
themselves, though.
To conclude, it seems that when an automatic tunneling mechanism is
being used, given an IPv4 address, the corresponding IPv6 address
could possibly be guessed with relative ease. This has significant
implications if the IPv6 security policy is less adequate than that
for IPv4.
Appendix B. IPv6 Privacy Considerations
The generation of IPv6 addresses of IPv6 addresses from MAC addresses
potentially allows the behavior of users to be tracked in a way which
may infringe their privacy. [RFC3041] specifies mechanisms which can
be used to reduce the risk of infringement. It has also been claimed
that IPv6 harms the privacy of the user, either by exposing the MAC
address, or by exposing the number of nodes connected to a site.
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Using stateless address autoconfiguration results in the MAC address
being incorporated in an EUI64 that exposes the model of network
card. The concern has been that a user might not want to expose the
details of the system to outsiders, e.g., fearing a resulting
burglary if a thief identifies expensive equipment from the vendor
identifier embedded in MAC addresses.
In most cases, this seems completely unfounded. First, such an
address must be learned somehow -- this is a non-trivial process; the
addresses are visible e.g., in web site access logs, but the chances
that a random web site owner is collecting this kind of information
(or whether it would be of any use) are quite slim. Being able to
eavesdrop the traffic to learn such addresses (e.g., by the
compromise of DSL or Cable modem physical media) seems also quite
far-fetched. Further, using RFC 3041 addresses for such purposes is
straightforward if worried about the risk. Second, the burglar would
have to be able to map the IP address to the physical location;
typically this would only be possible with information from the
private customer database of the ISP and, for large sites, the
administrative records of the site.
B.2 Exposing Multiple Devices
Another concern that has been aired involves the user wanting to
conceal the presence of a large number of computers or other devices
connected to a network; NAT can "hide" all this equipment behind a
single address, but is not perfect either [FNAT].
One practical reason why some administrators may find this desirable
is being able to thwart certain ISPs' business models. These models
require payment based on the number of connected computers, rather
than the connectivity as a whole.
Similar feasibility issues as described above apply. To a degree,
the number of machines present could be obscured by the sufficiently
frequent re-use of RFC 3041 addresses -- that is, if during a short
period, dozens of generated addresses seem to be in use, it's
difficult to estimate whether they are generated by just one host or
multiple hosts.
B.3 Exposing the Site by a Stable Prefix
When an ISP provides IPv6 connectivity to its customers, it delegates
a fixed global routing prefix (usually a /48) to them.
Due to this fixed allocation, it is easier to correlate the global
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routing prefix to a network site. In case of consumer users, this
correlation leads to a privacy issue, since a site is often
equivalent to an individual or a family in such a case. That is,
some users might be concerned about being able to be tracked based on
their /48 allocation if it is static [I-D.dupont-ipv6-
rfc3041harmful].
This problem remains unsolved even when a user changes his/her
interface ID or subnet ID, because malicious users can still discover
this binding. This problem can be solved by untraceable IPv6
addresses as described in [I-D.ietf-v6ops-nap].
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